The present invention relates to photoresist patterning. More particularly, the present invention relates to strengthening a photoresist pattern against damage.
Photolithography for patterning photoresist is widely used in the production of semiconductor devices. Presently, 248 nm lithography, using a Krypton Fluoride (KrF) light source, and higher wavelength lithography are very common. It is desirable to perform photolithography with light of a wavelength less than 248 nm to allow a reduction in the design rules used to create smaller semiconductor devices. 193 nm lithography using an Argon Fluoride (ArF) light source may be used commercially obtain 0.1 μm to 0.07 μm sizes. Also, 157 nm lithography using a Fluorine (F2) light source may also be used in the future.
Chemically amplified positive resists are a type of photoresist that may be used. Amplified positive resists may comprise a polymer with a functional group combined with a separate photoacid generator molecule. Upon exposure to a certain light, the photoacid generator may create a weak acid, which diffuses through the polymer material. After the exposure, photoresist material may be baked, which may cause the acid to attack certain cleavable groups, which deprotects those groups and leaves a carboxylic acid in their place. The photoresist layer may then be developed by a solvent in a wet bath, which binds to the carboxylic acid, which makes polymers with carboxylic acid groups soluble in the solvent while polymers without the carboxylic acid groups are insoluble in the solvent.
For photoresist materials used for 248 nm and higher wavelength lithography, cross-linking of the photoresist polymer material is typically induced by exposure to deep UV radiation. This method of cross-linking may not be effective for photoresist material used for 193 nm and lower wavelength lithography, such as 157 nm and 45 nm, because these materials are designed to only weakly absorb deep UV radiation. Generally, 193 nm and lower wavelength lithography photoresist material will require the absence of double-bonded carbon and aromatic carbon groups in the polymer. These functional groups have traditionally been used as sites which can be activated to induce cross-linking in photoresist, in some cases by exposure to deep UV radiation, to improve etch and ion implantation resistance. It is believed the absence of these functional groups in 193 nm and lower wavelength photoresist materials, reduces the possibilities for cross-linking these polymers, for example when exposed to deep UV radiation.
Current chemically amplified photoresist material developed for use with 193 nm and lower wavelength lithography is adversely affected by plasma etching or ion implantation. Exposure of a 193 nm or lower wavelength photoresist film to an etch plasma may lead to a roughening of the film surface and a resulting degradation of the pattern quality. Striations in the walls of trenches and vias, an increase or decrease in critical dimensions, distortion of feature shapes, and pinhole etching of dielectric beneath photoresist film may be some of the undesirable outcomes of this degradation. The release of functional groups during plasma processing may occur from the bulk of the photoresist layer, which may significantly modify the plasma and may affect the etch chemistry. The release of these functional groups may also cause some of the above-described roughening of the film. Photoresists designed for 193 nm and lower wavelength lithography may also be etched at higher rates, compared to established photoresist materials.
The 193 nm and lower wavelength lithography photoresist film may also be degraded during the ion implantation process, due to direct interaction of ions or heating of the photoresist film. Typical high-throughput conditions for ion implantation result in significant wafer heating, which may cause thermally induced reticulation or roughening of the photoresist film and degradation of the photoresist pattern quality. Degradation of the photoresist pattern quality during implantation can lead to several undesirable outcomes, including poor CD (critical dimension) control of the implanted region, reduction in the absolute dosage, and modification of the ion depth profile.
To obtain desired critical dimensions, such resist thicknesses may be kept in the range of 0.3 to 0.5 microns. The use of such thicknesses may require improved etch selectivity over what current technology commonly provides.
In view of the foregoing, it is desirable to provide a photoresist film for 193 nm and lower wavelength lithography that is more resistant to damage caused by plasma etching and ion implantation, is less susceptible to shrinkage, and is etched at a reduced rate to provide improved etch selectivity.